Steerable or phased array antennae are well known in the art in both the electromagnetic and the ultrasonic acoustic fields. They are less well known in the sonic (audible) acoustic area.
The commonly-owned published International Patent application No WO 01/23104 describes sonic steerable or phased array antennae and their use to achieve a variety of effects. The application describes a method and apparatus for taking an input signal, replicating it a number of times and modifying each of the replicas before routing them to respective output transducers such that a desired sound field is created. This sound field may comprise a directed beam, focussed beam or a simulated origin.
Control of direction and beamwidth, i.e. the steerability, of a beam is required to generate and steer broadband acoustic signals, such as multi-channel audio signals. These parameters depend on the frequency or range of frequencies of the emitted signal. In addition they depend on the spatial arrangement of the emitting sources. The spatial arrangement in turn is subject to technical constraints arising from the technical properties of the transducers employed and costs. Thus, the design of a functional and economically viable source of acoustic energy capable of projecting sound into predetermined directions, in short herein referred to as digital loudspeaker system or DLS, is a complex task.
In WO 01/23104 the direction of a beam is controlled by delaying the output of each transducer across the array. Appropriate delays, which are frequency dependent, lead to a constructive interference at a predetermined location of all the signals as emitted from the transducers of the array.
On the other hand, the beamwidth—whether measured as the angular distance between two minima or by any other known definition—is in the simplest case a function of direction of the beam, its frequency and the emission area or width of the array of sources from which the beam emanates. For previously-described arrays, the beam becomes narrower with increasing frequency. With broadband signals, spanning a broad range of frequencies, potentially many octaves in case of audio signals, this makes it difficult to generate and steer a beam at the lowest frequency components of the signal. One way to overcome this problem is by extending the lateral dimensions of the array of the antennae. However, such larger array narrows the beam at high frequencies. This effect could be disadvantageous in practical applications such as, for example, the projection of sound.
It is therefore an object of the invention to improve the ability of an array of acoustic transducers to emit and steer beams of broadband sonic signal while minimizing mechanical and electronic components required for its implementation.
It is another object of the invention to obtain an array of broadband transducers that emits broadband wave signal with sufficient directivity at low frequency and sufficient beamwidth at high frequencies.
It is a further object of the invention to obtain an array of broadband transducers with improved steerabilty of sound beams having different travel paths before reaching a listener.